This invention relates to a hydromechanical infinitely variable transmission. It provides for improved utilization of vehicle engine power by enabling the engine to operate within a narrow speed range which has been optimized for minimum emissions, maximum fuel economy and maximum power, regardless of vehicle operating conditions.
When utilizing conventional torque converter or manual transmission, many compromises are imposed upon the engine because it must provide adequate performance over a wide range of torque and speed. The practice of most vehicle manufacturers of providing a selection of optional axle ratios for the vehicle is only one of the many attempts which have been made to reduce the compromise for any given application.
With an infinitely variable transmission, the engine can always be operated in a speed range in which it is capable of producing rated power. Therefore, vehicle performance in any given application can be maintained or even improved while utilizing a smaller engine. However, infinitely variable transmissions of the pure hydrostatic type are limited to applications where significant power losses can be tolerated in return for the benefits of improved transmission ratio control.
Hydromechanical transmissions offer the control benefits of a hydrostatic transmission, and, since only a portion of the engine power is transmitted by the hydraulic units, they provide a means of removing the performance barrier of excessive power losses. The extent to which any hydromechanical transmission can accomplish this is a function of the percentage of power which must be transmitted hydraulically.
The transmission of this invention can transmit high horsepower over a wide range of output speed variation at a constant input speed and horsepower. It is a planetary-hydraulic device which differs from the previous transmissions in its ability to transmit power over a wide range with a minimum of transmitted hydraulic horsepower, and a minimum of installed hydraulic horsepower. It provides full engine braking over its entire range of operation.
The invention avoids the pitfalls of excessive complexity, speeds or loads in the gear train. Maximum reliablility and minimum cost have been obtained by utilizing standard commercial hydraulic units which are operated totally within their long-life rated conditions of speed and power. In addition, the clutches can utilize the same low-cost paper elements presently employed in high production automobile torque converter transmissions. For a comparable power rating, a smaller number of elements than in a torque converter power shift transmission can be utilized, because at all shift points the clutch elements are virtually synchronous. The number of elements is, therefore, a function not of their thermal capacity, but of their steadystate torque capacity.
Among the fields of applications of the transmission of this invention are passenger and competition cars, highway and off-highway trucks, buses, agricultural and construction equipment, military vehicles, and industrial drives and machine tools.
The invention makes it possible to design transmissions of an extremaly wide range of speed and torque variation at full power. This is required in construction and off-highway equipment, for example, where torque multiplication of the order of 18:1 and 24:1 may be encountered. Machine tool drives may require even wider ranges, and they are possible.
The use of this transmission in a piston engine vehicle enables reduction of exhaust emissions and improvement in the specific fuel consumption by programming the engine to operate within its optimum range under all road conditions without regard to transmission torque output requirements. Both the NOx and CH emissions can be minimized by optimizing the engine for operation in a specific narrow range. In addition, a smaller engine may be utilized for any application, as the transmission enables full engine power to be developed at any vehicle speed except for the lower speeds where the vehicle is traction limited. It is particularly desirable to operate turbocharged diesel engines in a narrow range of speed.
The same considerations listed for piston engines also apply to rotary combustion chamber engines. The benefit in reduction of CH emissions is of a much greater magnitude, however, due to the high rate of change in emission characteristics for rotary combustion engines with respect to engine speed.
The benefits to be derived from the application of this transmission to a gas turbine are also significant. Manufacturing cost is a major drawback in producing a turbine today. This, to a large degree, is a function of the complexity required in the design of a turbine for use under the varying torque and speed conditions of a road vehicle. With the hydromechanical transmission of this invention, the turbine can be programmed to operate only under those conditions during which it is most efficient. Therefore, a single-shaft turbine becomes feasible, as it is more economical to manufacture than the two-shaft design normally proposed for vehicle application. Since constant speed operation is feasible, the problems in connection with the throttle response time of a turbine do not arise. Because there is an infinite variation in speed and torque in the transmission, and no interruption of power flow occurs at any time, the turbine is never unloaded.